Amplifiers are typically used in communication transceivers for the amplification of weak electrical signals. Two main concerns associated with amplifier design are: (1) the minimization of noise added to the signal by the amplifier; and (2) achieving maximum power transfer between a source producing the signal and the amplifier.
In order to achieve maximum power transfer between the source and the amplifier, the input impedance (Z.sub.IN) of the amplifier must be equal to the complex conjugate of the source output impedance (Z.sub.S), namely, Z.sub.IN =Z.sub.S '. In the case where the output impedance of the source is real (Z.sub.S =R.sub.S), then the input impedance of the amplifier must also be real (Z.sub.IN =R.sub.IN). For maximum power transfer between the source and the amplifier, R.sub.IN =R.sub.S. This is commonly referred to as "power-matching".
The noise added to the signal by the amplifier results in a degradation of the signal-to-noise ratio (S/N) at the output of the amplifier. A figure of merit for the amount of noise added by the amplifier is the ratio of the signal-to-noise ratio at the input (S/N).sub.IN to the signal-to-noise ratio at the output of the amplifier (S/N).sub.OUT. This ratio is commonly referred to as the noise factor (F) of the amplifier, and is used to calculate the noise figure (NF) of the amplifier according to the formula NF=10 log.sub.10 (F), where F=(S/N).sub.IN /(S/N).sub.OUT.
Typically, amplifiers are integrated as monolithic ASICs (Application Specific Integrated Circuits). Maintaining a power-matched condition very accurately over production tolerances of the components that are used in the amplifier has traditionally been difficult. This especially presents a problem when the power-matched amplifier is used at the output of a filter, e.g., crystal filter, SAW filter, etc., whose components typically have a high sensitivity with respect to changes in the source output and termination (amplifier input) impedances. Accordingly, maintaining a proper power-matched input impedance of the amplifier following the filter over temperature and process spread of the components, while at the same time minimizing the noise figure of the amplifier, is critical for the overall performance of the system in which these components are utilized. Generally, an improvement in one area has resulted in a penalty in the other area.
The present invention is directed toward overcoming one or more of the above-mentioned problems.